The National Park System is threatened by heavy use, overdevelopment of surrounding areas and inadequate research, a report issued by an environmental group says.

The Conservation Foundation report released today also says that the nation's 334 parks and historic monuments are plagued by the decline of plant and animal species within the parks.

"The National Park System has entered an era for which its traditions and policies have not prepared it," the nonprofit foundation says.

The new director of the National Park Service, William Penn Mott Jr., said yesterday that he generally agrees with the study.

"What really drives me is this sense that we don't know the long-range impact of our human activities in the park today," he said.

In a speech prepared for delivery last night before the Greater Yellowstone Coalition at Yellowstone National Park, Mott said, "We must not and should not use up the resource or diminish its inherent natural and historic values.

"When in any doubt, we must err on the side of preservation," he said. "Should we subsequently find ourselves wrong, we can always provide for more use."

The Conservation Foundation report said the impact of heavy use and deferred maintenance, combined with encroachment by landholders near parks, "will seriously damage the parks unless checked at least in part by an infusion of fix-up funds and by new imaginative responses."

Mott has said he will close the gates of some major parks once they have reached their capacity of visitors.

The foundation recommends a 10-year, $500 million effort to restore park resources and a 10-year, $2 billion program to acquire land around and in parks.

But Mott said yesterday that the Park Service budget will be tight for some time.

Scores of scientists, environmentalists and government officials surveyed last week said Mott faces many of the problems outlined five years ago in a Park Service report on threats posed to the parks.

That report listed 73 different threats grouped in seven categories -- aesthetic degradation, air pollution, water pollution, the impact of visitors, removal of resources, encroachment by exotic wildlife and the effect of park operations.

It concluded that "current levels of science and resource management are completely inadequate to cope effectively with the broad spectrum of threats and problems which have been identified and discussed in this report."

While both the quality and quantity of research has since increased, "it is not sufficient," according to John Reed, chief of the Biological Resources Division of the Park Service.

Clay Peters of the Wildlife Society said the Park Service had "abysmally failed" to catalogue threats to the parks. "It has just not been a high priority," he said.

Peters' view was echoed by Ro Wauer, assistant superintendent at the Great Smoky Mountains National Park. "We're not getting any request for what's going on," he said. "The Washington office is presumably not interested in what is happening or how things might be . . . . This park will continue to go downhill."

Proposals in the last two Congresses for a biannual report on the state of the nation's parks have failed.

"The people who have been getting away with certain things adjacent to the parks do not want the right hand to know what the left is doing," Rep. John F. Seiberling (D-Ohio) said. "They view it as an effort to have more government control. In a way, if the government knows more, then I suppose they can exercise more control."